The present invention relates to a tone generator wherein tones are produced by reading out values contained in one of a number of arrays stored in an EPROM. Each of these arrays contains a series of binary representations of the waveform to be generated; however, these values have been adjusted to compensate for distortion that may arise in subsequent processing of the tone, such as to account for the performance characteristics of subsequent amplifiers speakers, or headphones as a function of the amplitude and frequency of the tone. A microprocessor is used to select among the various arrays stored in the EPROM based on the amplitude and frequency of the desired tone.
The prior art contains a number of examples of waveform generators by reading out values stored in a memory, such as Niimi, U.S. Pat. No. RE. 31,004, "Electronic Musical Instrument Utilizing Data Processing System" at column 5, line 59 through column 6, line 10; and Tomisawa, U.S. Pat. No. 4,036,096 "Musical Tone Waveshape Generator", at column 1. In addition, various digital techniques have been used in the electronic music arts, specifically electronic organs for many years. The frequency of the tone produced by such devices is determined by the rate at which the waveform is read out of the memory.
All electrical or physical devices that generate, process, or use a tone inevitably result in some distortion of the tone. For example, all amplifiers and speakers have performance characteristics that vary both as a function of the amplitude and frequency of the tone being reproduced. In many applications this distortion is of little or no practical significance. However, in other fields, such as audiometric testing, this distortion is of vital concern and must be eliminated to the fullest extent possible. A wide variety of conventional manual calibration and equalization techniques have been employed in audiometric testing equipment to minimize such distortion. This approach has the disadvantage of adding substantially to the complexity, costs and weight of the audiometer.